Influence of multidentate N-donor ligands on highly electrophilic zinc initiator for the ring-opening polymerization of epoxides

A set of multidentate ligands have been synthesized and used to stabilize the putative highly electrophilic zinc species initiating ring-opening polymerization (ROP) of cyclohexene oxide (CHO) and propylene oxide (PO). Reaction of the bidentate C-2-chiral bis(oxazoline) ligand ((BOX)-B-R2,R3: R-2 = (4S)-tBu, R-3 = H (a); R-2 (4S)-Ph, R-3 = H (b); R-2 = (4R)-Ph, R-3 = (5S)-Ph (c)) with Zn(R-1)(2) (R-1 = Et (1), Me (2)) led to the heteroleptic three-coordinate complexes ((BOX)-B-R2,R3)ZnR1, 1a-c and 2a, which were isolated in 92 -96% yield. Next, two pyridinyl-functionalized N-heterocyclic carbene (NHC) ligands have been designed and synthesized: the 1,3-bis(2-pyridylmethyl) imidazolinium salt (d) and the protected NHC adduct 2-(2,3,4,5,6-pentafluorophenyl)-1,3-bis(2-pyridylmethyl)imidazolidine (e). The reaction of ligands d and e with ZnEt2 led directly to the formation of (NHC)ZnEt(Cl) 3d complex with ethane elimination and the adduct (NHC-C6F5(H))ZnEt2 4e, respectively, in high yield. In situ combinations of selected complexes 1a-c, 3d and 4e with B(C6F5)(3) (1 or 2 equivalents) give active systems for ROP, with high productivity (3.3-5.9 10(6) g(polym.) mol(Zn)(-1) h(-1)) and high molecular weight (M-n up to 132 10(3) g mol(-1)) for CHO polymerization. Although the in situ B(C6F5)(3)-activated zinc species were not isolated, the sterically demanding BOX ligands (1c > 1b > 1a) and functionalized NHC ligands seem to enhance the stability of highly electrophilic zinc complexes over ligand redistribution, allowing a better control of the cationic ROP as reflected particularly for 3d and 4e complexes by their respective efficiency (42-88%). (C) 2011 Elsevier B.V. All rights reserved.